Something draining battery in Windows 10 mobile?

For all the talk about improved battery performance, I'm now even more disappointed with my HTC One M8. To begin, here's the background of my story. Microsoft and HTC abandoned us, but I stuck out with the Insider builds. It seemed things were OK for a while, until the Build conference where I finally decided to no longer rely on my HTC as a daily driver and bought the iPhone 6s plus. Almost everyone said that Apple skimped on the battery capacity in their phablet phone, but after more than a month I have never gone below 40% capacity. I can leave this phone unattended overnight and come back only to have 1% of battery usage gone.

Now I have kept my HTC as a backup (I have 2 lines). And even though I no longer use it and pretty much have casual glances at it, I'm appalled at the battery life. The thing will lose 10% just sitting there overnight. Why? What's Microsoft doing wrong here? (I am running the latest release preview build, and have even done soft reset). Even windows phone 8.1 before did not drain that much with no action at all. I hate to admit it but perhaps Apple really has a lot of good stuff going on with their "skimpy" battery after all.

Check settings >system > battery > which apps are using the most energy?

Again, in battery section, which apps can run in the background? Can you disable those you don't need? Keep messaging enabled to run in background

In battery section, you can also set up battery saver to kick in when the battery reaches a certain percentage. I keep mine on 100% so it's always on battery saver mode

In mail app, how often do you have mails to sync? Generally the longer the time period, the less the battery usage

What about brightness settings? Is it on high or auto? Auto is better than a fixed setting

If you finds that messaging app uses a lot of battery, this could be because it's linked with skype. Go to messaging app, settings and unlink skype

In photos app, do you have auto upload of photos enabled? If yes, try to not upload high res photos and videos unless you are charging the phone and on wifi

When you use some apps that are not data heavy like whatsapp or mail, you can use a lower network speed like 2G. It's what I do usually. I only use LTE when I have to use 6tag

If you're using edge for basic browsing, you could try using UC browser. Not only does it use less data but you can also disable images which means you save even more on data. The less data your phone has to receive, the less the energy used

In settings > privacy - check each of those settings to see which apps can use what. For example I can't understand why Word would need to access location info so I disabled that. There might be apps using services that they are not meant to use

Finally, keep wifi/cellular off when you don't need to have them on. The toggles in action centre are pretty fast so it doesn't take much time in switching internet on/off

Disable auto update of apps, you only update when you are ready and not when you are in need of critical things